The present invention generally relates to pistons for internal combustion engines and more particularly relates to a piston bowl formed in the head of a piston for a diesel engine having offset-mounted fuel injectors.
In diesel engines, high compression ratios are reached in the cylinders during a piston compression stroke. Fuel injected into the cylinder during this compression atomizes and combusts, creating a spray plume. Spray plume impingement (contact with a surface of the combustion chamber) is undesirable because it causes improper combustion, resulting in black smoke exhaust. Also, impingement creates extraordinarily high thermal stresses and fatigue on the region of impinged piston material, ultimately causing piston failure. Accordingly, adjacent a fuel injector, a compressed cylinder must provide a volume having suitable size and shape to accommodate the spray plume without impingement. An appropriately designed cylinder volume facilitates complete atomization and combustion of the injected fuel during compression by providing an adequate distance for spray plume travel to prevent impingement.
To provide a suitably shaped combustion volume, a diesel engine piston is conventionally provided with an annular piston bowl or recessed cavity in the piston head. The position and depth of the piston bowl is dictated by factors including the position of the corresponding fuel injector, injection pressures, nozzle configuration, and resulting spray plume size and shape. At the same time, the total volume of a piston bowl is desirably minimized in order to permit high compression.
Piston bowls are conventionally formed by machining the piston with a lathe. By nature, the conventional process of "turning" a piston on a lathe results in a bowl surface which is symmetrical and uniformly shaped around an axis of rotation, thereby resulting in a circular bowl shape described herein as an "annular." A conventional piston bowl is "annular" or circular in cross-section as intersected by a plane parallel to a top of the piston.
A typical fuel injector nozzle tip is configured to inject a plurality of fuel streams which are directed radially-outwardly from the tip. Therefore, a conventional piston bowl is typically formed to include a raised central portion or "apex" or "pip" which fills in space directly under the injector that is not needed to accommodate the spray plumes. The bowl apex is desirably centered on an axis under the fuel injector nozzle because a fuel injector tip injects the fuel (and spray plumes) in a more radial than downward direction.
The development of sophisticated, electronically-controlled, high-pressure fuel injection systems for modern diesel engines has been motivated by increasingly stringent emissions regulations. In order to achieve lower emission levels, fuel system developments have led to higher peak injection pressures as well as a wider range of injection timings. Higher injection pressures result in higher fuel spray velocities within an engine cylinder and, accordingly, increased spray plume penetration distance.
In order to avoid impingement of larger spray plumes on piston surfaces, pistons have been designed with increasingly large piston bowl diameters. By increasing bowl diameter, a greater distance is available to accommodate the larger spray plumes. Ideally, as a result of the increased bowl diameter, a sufficient space is provided for the injected fuel to atomize completely combust before reaching a surface of the combustion chamber, i.e., the piston head.
"Offset" piston bowls are known for use in engines having offset fuel injectors. For example, special bowl design difficulties arise in engines utilizing two-valve cylinder head designs (i.e., one intake valve, and one exhaust valve). In order to desirably maximize valve port areas in a two-valve cylinder head, geometry constraints dictate that the intake and exhaust ports extend closely toward each other, substantially occupying the center of the cylinder bore. The fuel injector must, therefore, be mounted in an offset-from-center location. Because the piston bowl is ideally centered under the offset injector nozzle tip, the piston bowl is correspondingly offset as well. Known "offset" piston bowls are annular in shape, and offset bowls are typically formed by "turning" the piston on a lathe along an offset axis.
In engines with offset injectors, a diameter of the annular offset piston bowl has been increased to accommodate increasing spray plume distances for the reasons explained above. Unfortunately, the proximity of the bowl surface to a side of the piston is limited by the structural limits of the piston, restricting the size of a possible annular bowl diameter. Too thin of a piston structure at some point near the bowl could result in piston failure from stress and fatigue. Thus, the diameter of a conventional offset piston bowl is limited so that the piston limits are not exceeded.
In an attempt to accommodate greater spray plume distances, "partially offset" bowl designs have been attempted in an offset piston bowl has an increased diameter, but wherein the bowl position is compromised as being offset relative to the center of the piston, but not optimally centered under the injector either. This unfortunately results in an position of the bowl apex which is offset relative to the injector (the apex is not centrally aligned with the aim of the injector), resulting in impingement on the apex.